A known digital signal receiving apparatus is disclosed in Japanese Patent No. 2001-4736A (hereinafter, referred to as reference 1, see FIG. 4) for example. FIG. 5 is a block diagram illustrating a general configuration related to a digital signal receiving apparatus 70 disclosed in the reference 1. As illustrated therein, the digital signal receiving apparatus 70 receives a desired signal S contained with a correlated noise signal N as an ambient noise, by an antenna 71. Then, the digital signal receiving apparatus 70 down-converts the received signal to a desired frequency band at a high-frequency processing portion 72. Further, the digital signal receiving apparatus 70 generates a corresponding digital signal (S″+N″) by converting the signal from analog to digital at an A/D converting portion 73 and outputs the digital signal to a noise-filtering unit 74.
In addition, the digital signal receiving apparatus 70 receives the noise signal N by means of another antenna 75. Then, the digital signal receiving apparatus 70 down-converts the received noise signal N to a desired frequency band at a high-frequency processing portion 77 of a reference noise signal-output portion 76. Further, the digital signal receiving apparatus 70 generates a reference noise signal Nr, which is correlated to the noise signal N, by converting the signal N from analog to digital at an A/D converting portion 78, and outputs the digital signal to the noise-filtering unit 74.
The noise-filtering unit 74 generates a noise cancel signal AN via an adaptive filtering-portion 79 on the basis of the reference noise signal Nr, and filters out the noise signal N by adding the digital signal (S″+N″) inputted from the antenna 71 to the noise cancel signal AN at an adder 80. A filter coefficient of the adaptive filtering portion 79 is successively updated so that the noise signal N (N″) is reduced to a minimum by the filter coefficient updating portion 81.
However, according to the digital signal receiving apparatus 70 of the reference 1, the antenna 75 and the reference noise signal-output portion 76 (the high-frequency processing portion 77 and the A/D converting portion 78) are required for receiving the noise signal N, thus leading to an increase of a size of a circuit. In this case, it is necessary not to input a desired signal S to the antenna 75. In other words, if the desired signal S is inputted to the second antenna 75, the desired signal S may be also generated as the noise cancel signal AN at the adaptive filtering portion 79 and the desired signal S may sometimes be filtered our or removed.
Another digital signal receiving apparatus has been proposed in Japanese Patent No. 2005-45314A (hereinafter, referred to as reference 2, see FIG. 1). The reference numerals described below related to Reference 2 are depicted in FIG. 1 of Reference 2. The digital signal receiving apparatus according to the reference 2 includes an adaptive filter (17), filter controlling means (18c) and demodulating means (18d). The adaptive filter (17) includes an adaptive mode and a non-adaptive mode. In the adaptive mode, a filter coefficient is successively updated and an ambient noise is filtered out in a signal-receiving standby state for receiving a desired signal. In the non-adaptive mode, updating of the filter coefficient is stopped in a signal-receiving state for receiving the desired signal. The filter controlling means (18c) controls switching of the adaptive mode and the non-adaptive mode of the adaptive filter (17). The demodulating means (18d) demodulates an output (correlation signal) of the adaptive filter (17) and generates a demodulated signal. The adaptive filter (17) includes an adaptive filter portion (17a, 17b) and a filter coefficient-updating portion (17c, 17d). At the adaptive filter portion (17a, 17b), the filter coefficient is changed in accordance with a change of the ambient noise in the signal-receiving standby state. The filter coefficient updating portion (17c, 17d) successively updates the filter coefficient of the adaptive filter portion (17a, 17b) so as to reduce the ambient noise to a minimum. When detecting means (18) judges a possible presence of a desired signal in a situation where the apparatus is in a signal-receiving standby state, the filter controlling means switches the adaptive filter from an adaptive mode to a non-adaptive mode so as to stop updating the filter coefficient.
In such a case, in the signal-receiving standby states the adaptive filter successively updates the filter coefficient and filters the ambient noise, reacting to the adaptive mode. On the other hand, in the signal-receiving state for receiving the desired signal, the adaptive filter stops updating the filter coefficient reacting to the non-adaptive mode. In other words, in the non-adaptive mode, the adaptive filter filters the ambient noise by use of the filter coefficient updated for filtering the ambient noise in the adaptive mode. Accordingly, in the signal-receiving state for receiving the desired signal, even when the ambient signal and the desired signal have a correlation, the ambient signal is filtered by the adaptive filter and the desired signal is received appropriately. In addition, even when the ambient signal and the desired signal have a correlation, the ambient signal and the desired signal can be selectively received because the filter controlling means controls the switching of the adaptive mode and the non-adaptive mode. Accordingly, the size of the circuit can be reduced in comparison with a configuration of the circuit in which the ambient signal and the desired signal are separately received.
However, in this type of digital signal receiving apparatus, in a signal receiving state for receiving a desired signal, when a pulse-shaped interfering wave is contained, a correlation output (an error signal e2) of the adaptive filter, which contains therein a pulse-shaped interfering wave, is inputted to a detection-judging portion. In this case, the detection-judging portion may not judge detection accurately even in a signal receiving state, and may judge there is an absence of a detection of a desired signal. Here, because the adaptive filter is not accurately controlled (switching control between an adaptive mode and a non-adaptive mode), the adaptive filter erroneously learns a desired signal as an ambient noise, thereby disabling to demodulate a desired signal accurately.
A need thus exists for providing a digital signal receiving apparatus, which can accurately judge a detection of a desired signal and prevent an adaptive filter from erroneously learning a desired signal even when a pulse-shaped interfering wave is contained.